The field of the present invention relates to semiconductor photodetectors. In particular, a semiconductor photodetector is described herein that includes an internal reflector.
FIGS. 1A and 1B illustrate a generic configuration including a planar waveguide 120 on a waveguide substrate 101. A surface-mounted photodetector 110 is placed on the waveguide substrate 101 (either directly, or on alignment/support members thereon) for detecting optical power propagating from an output face of waveguide 120. FIGS. 1C and 1D illustrate another generic configuration including an optical fiber 150 received in an alignment groove 152 for illuminating a photodetector 110 (surface-mounted on the groove substrate 151, as in FIGS. 1C and 1D, or fabricated directly on the groove substrate). Reasons for using a photodetector in such circumstances are numerous. For example, the optical power propagating through waveguide 120 or fiber 150 may comprise an optical telecommunications signal modulated at high data rates (10 or more Gbits/sec, for example), and a high-speed photodetector 110 may be employed as a receiver for converting the optical signal into an electronic signal. In another example, the optical power propagating through waveguide 120 or fiber 150 may comprise a portion of the output of a semiconductor laser or other light source split from the main optical output for monitoring purposes. The resulting signal from the photodetector may be used for signal normalization, as a feedback control signal for stabilizing the operation of the light source, and/or for other purposes. In this type of application a high-speed photodetector may or may not be required. Many other circumstances may be envisioned wherein detection of optical power propagating through an optical waveguide or an optical fiber may be useful.
Silicon is a commonly-used planar waveguide substrate, typically provided with a silica buffer layer and one or more silica-based planar waveguides fabricated on the silica buffer layer (so-called Planar Waveguide Circuits, or PLCs). Such substrate may also be readily provided with grooves for receiving an end of an optical fiber. It is often the case (in telecommunications devices) that the wavelength of the optical power carried by waveguide 120 or fiber 150 lies in the 1.3 μm to 1.6 μm region, for which silicon-based photodetectors are not suitable. Photodetectors based on III-V semiconductors are suitable for this wavelength region, but the materials are not compatible for fabrication of the photodetector directly on a silicon or silica surface. Even if waveguide substrate and detector materials are compatible, it may nevertheless be desirable for providing the semiconductor photodetector as a separate component for later assembly for other reasons (incompatible processing steps, design flexibility, customization of waveguide and/or photodetector, and so forth). A separately fabricated semiconductor photodetector 110 (III-V or otherwise) is therefore often assembled onto substrate 101 or 151 (silicon or otherwise) and aligned for receiving and detecting at least a portion of the optical power propagating through waveguide 120 or fiber 150. The present disclosure addresses suitable fabrication and/or adaptation of semiconductor 110 for enabling and/or facilitating such assembly.
For mounting on a substantially planar substrate 101 or 151, it is advantageous for photodetector 110 to also be fabricated/mounted on its own substantially planar substrate. The light to be detected propagates substantially parallel to these planar substrates. However, the layers that form the photodetector active region on the substrate are also substantially parallel to the substrates, rendering absorption and detection of the light by the photodetector problematic in many cases. Redirection of the light out of a plane parallel to the substrates facilitates detection thereof. A photodetector implemented according to the present disclosure employs internal reflection from an angled face of the photodetector substrate for directing the light toward the active region thereof.